The invention relates to a method for nondestructive testing of workpiece surfaces by a fluorescent penetration test.
There are known methods of nondestructive testing of workpiece surfaces. In the so-called penetration test, a penetrant is applied to a cleaned workpiece surface to be inspected. The capillary action of fine surface cracks and pores facilitates the penetration of the penetrant into such recesses in the workpiece surface. The so-called fluorescent penetration test works with a fluorescent penetrant. After a predetermined contact time, excess penetrant is then washed off in an intermediate cleaning operation. Next a developer is applied to the workpiece surface to be inspected. The developer promotes rewetting of the penetrant on the recess of the workpiece surface, so that the developer causes the penetrant to be drawn out of the recess and to the surface. This permits clear visualization of possible irregularities in the workpiece surface, e.g., crack-like separations of material.
The fluorescent penetration test is used in the aeronautical, shipping and automotive engineering industries as well as in metalworking industries in particular. However, other substances such as ceramics can also be inspected for surface cracks and pores accordingly.
However, one disadvantage of the known method for use in aeronautical engineering in particular is that the corresponding areas of the presumed recesses must be surface cleaned manually after application of the developer and after the corresponding mapping of the recesses and/or surface cracks in order to verify the display and remove excess penetrant that has been redistributed on the workpiece surface around the area of the recess by application of the developer. The penetrant, forming a lining only on the recess and/or the surface crack, is visualized only after this mechanical cleaning. An accurate evaluation and measurement of the dimensions of the recess are made possible only in this way. However, this manual cleaning step is time-consuming and cost-intensive. Furthermore, it is very difficult to standardize, so that a standardized classification of possible recesses and/or defects in the workpiece surface is also difficult.
The object of the present invention is to create a method for nondestructive testing of workpiece surfaces which will permit faster and less expensive testing of workpiece surfaces.
The method according to an embodiment of the invention for nondestructive testing of workpiece surfaces by a fluorescent penetration test comprises the following steps:                a) cleaning the area of the workpiece surface to be inspected;        b) applying a fluorescent liquid penetrant to the area of the workpiece surface to be inspected, wherein the penetrant penetrates into possible recesses in the workpiece surface;        
c) removing the excess penetrant from the workpiece surface;
d) applying a developer to the area of the workpiece surface to be inspected;
e) bleaching the fluorescent penetrant by a beam of light in the layer formed by application of the developer to the workpiece surface; and
f) visual evaluation of the fluorescent penetrant remaining in the existing recesses in the workpiece surface.
Manual cleaning of the workpiece surface after application of the developer may be omitted through bleaching of the fluorescent penetrant in the developer layer. The corresponding recesses and/or defects, crack-like separations of material, pores and cracks in the workpiece surface are clearly delineated, marked and mapped due to at least partial bleaching of the penetrant, which has been drawn out of the corresponding recesses by the developer and back to the surface of the workpiece. The visual evaluation is performed on the basis of the fluorescent penetrant which is still in the aforementioned recesses, so that the recesses are clearly delineated with respect to the surrounding workpiece surface. Furthermore, some of the penetrant in the recesses is forced back into the developer layer above and/or next to the corresponding recesses due to the osmotic pressure. This advantageously yields a type of magnification effect, which emphasizes the recesses more clearly and makes them recognizable. In addition, some of the penetrant that has not collected in the recesses but instead is adhering to irregularities in the surface of the workpiece or foreign bodies on the surface is reliably bleached, so that the remaining unbleached portions of penetrant clearly indicate the presence of recesses. Manual cleaning of the surface is not necessary. The method according to the invention can be standardized to advantage, thus permitting a classification of possible defects and/or recesses on the workpiece surface which is also standardized. In addition, the method according to the invention can be performed easily, quickly and inexpensively.
In an advantageous embodiment of the method according to the invention, a first visual evaluation of the workpiece surface is performed in a process step d1) prior to the process step e), i.e., bleaching of the fluorescent penetrant. Due to this intermediate step, a first visual inspection of possible recesses and/or defects in the workpiece surface can be performed, so the subsequent bleaching of the fluorescent penetrant is concentrated in the areas of the workpiece surface where possible recesses and/or defects are detected. This may contribute toward better acceleration of the course of the process.
In additional advantageous embodiments of the method according to the invention, the visual evaluation according to process steps f) and/or d1) may be performed by bombarding the workpiece surface to be inspected with UV light. Recesses in the workpiece surface that have been filled and/or wetted with the fluorescent penetrant can be detected distinctly in this way. In addition, it is possible to use an intense UV light for the bleaching process according to process step e). The radiation intensity of the UV light used for bleaching according to process step e) is usually greater than the radiation intensity of the UV light used in process step f) and/or d1). In addition, there is the possibility that the wavelength of the UV light used in the process step e) corresponds to that in process step f) and/or d1). A distinct and accurate visualization of recesses in the workpiece surface is possible due to the UV light used on the one hand, while on the other hand the fluorescent penetrant in the layer formed by applying the developer to the workpiece surface is reliably bleached so that in particular portions of the penetrant that have not accumulated in the recesses but instead are adhering to irregularities in or foreign bodies on the surface of the workpiece are reliably bleached. If UV light of the same and/or a comparable wavelength is used for the visual evaluation and for the bleaching process, then a single UV light source may be used.
In an additional advantageous embodiment of the method according to the invention, the process steps b) through f) are repeated after the end of the process step f). Repeating these process steps may advantageously lead to an even more accurate mapping of unwanted recesses in and/or on the workpiece surface.
In an additional advantageous embodiment of the method according to the invention, the workpiece surface is a surface of a turbo engine, in particular a component of a gas turbine. Components of turbo engines, e.g., an aircraft jet, are especially critical with respect to irregularities on the surfaces of the components, e.g., fine cracks.
In an additional advantageous embodiment of the method according to the invention, the penetrant is a fluorescent dye penetrant. By using a fluorescent dye penetrant, in addition to the display of fluorescence, a color representation of possible recesses in the workpiece surface is also possible. The visual evaluation of the corresponding recesses in the workpiece surface can therefore be performed very easily and very accurately.
The method according to the invention described above is used in particular in the production, final testing and servicing of components of turbo engines, in particular components of a gas turbine.
Additional features of the invention are derived from the claims, the exemplary embodiments and the drawings. The features and combinations of features mentioned in the description as well as the features and combinations of features mentioned below in the exemplary embodiments may be used not only in the particular combination indicated but also in other combinations or alone without going beyond the scope of the present invention.